UBM layers may be dry or wet etched to prevent shorts. Wet etching may be preferred over dry etching because no additional cleaning of etch residue is required. Wet etching costs also are less than dry etching. However, wet etching etches beneath mushroom plated metal bumps causing an undercut.
More specifically, UBM wet etching removes exposed metal layers that are masked either by a photoresist, such as in a solder paste process, or that are masked by a metal bump, such as in an electroplated solder process. Two issues that are addressed for a successful UBM etch are complete metal layer removal where left exposed and control of the undercut caused by undesirably removing the UBM layer from beneath the metal bump.
Current wet etching solutions that include sulfuric acid (H2SO4), hydrogen peroxide (H2O2), and water, followed by dilute hydrofluoric acid (HF) cause 2.5 to 3 microns of undercut of the metal bumps. Such a large amount of undercut reduces the stability of the metal bump. Further, the current wet etching solutions also attack layers of the metal bump, such as nickel or aluminum layers.
A need, therefore, exists for a method of wet etching a UBM layer that creates a smaller undercut of a metal bump and the resulting device.